In an image forming apparatus which prints an image by transferring a developer image (toner image), formed on the surface of a photosensitive drum as an image carrier, onto a transfer material such as a paper sheet which is a transfer medium (printing medium), the transfer material is passed through a transfer portion formed in a contact portion between the photosensitive drum and a transfer member such as a transfer roller urged against the photosensitive drum. A voltage is applied to the transfer member in synchronism with the timing of the passage, and the toner image on the surface of the photosensitive drum is transferred onto the transfer material by the action of the electric field formed by the voltage. An image forming apparatus of this type has been put into practical use.
Also, with the recent progress of an information-oriented society, the needs for color printers are increasing, and an inline type of printer is attracting attention. In this inline type of printer, a plurality of image carriers corresponding to different colors are arranged in a line in order to increase the color image output speed. The inline type of printer forms toner images of these different colors in turn by using the individual image carriers, and transfers the toner images onto a transfer material directly or via an intermediate transfer member.
In the conventional printers as described above, when a user is continuously printing images having a low printing ratio (a low pixel ratio per page) over a long time, deterioration of toner progresses as the number of printed sheets increases, and the amount of high-triboelectrification toner which is charged up more strongly than normal toner increases in a developing device. This is so because the toner in the developing device stays in it for long time periods while the toner repetitively undergoes triboelectrification by a developing sleeve (developing roller) or by an elastic blade in contact with the sleeve. This strongly charged toner increases its electrical adhesion to individual members, and therefore worsens the properties of development onto the photosensitive drum or the properties of transfer onto the intermediate transfer belt or transfer material. This decreases the density of solid images and the like.
In this strongly charged toner, the electrical repulsion force between toner particles also increases. This deteriorates the image quality by scattering and the like when line images or characters are transferred. For example, images are recently often printed on glossy paper and the like especially in color machines. In a case like this, the image quality is increased by increasing the glossiness of images by lowering the fixing speed of the apparatus. Since glossy paper has improved toner fixing properties, the area of spread toner increases. If line images or characters scatter, therefore, this scattering is conspicuous on glossy paper, although it may be inconspicuous on plain paper. In addition, a high whiteness of glossy paper makes scattering conspicuous, and this increases the thicknesses of line images and characters and spreads thin lines, thereby deteriorating the image quality. These phenomena are particularly notable after images are continuously printed in a low-temperature, low-humidity environment.
In contrast, when a user is continuously printing images having a high printing ratio over a long time by using the conventional printer described above, deterioration of toner progresses as the number of printed sheets increases. Consequently, the amount of low-triboelectrification toner which is charged up more weakly than normal toner or the amount of reverse-polarity-triboelectrification toner which is charged up to the polarity reverse to that of normal toner increases in the developing device. This is so because the toner in the developing device is successively discharged outside the developing device without being much affected by triboelectrification by the developing sleeve or by the elastic blade in contact with the sleeve. This weakly charged toner or reverse-polarity toner reduces the electrical adhesion to the developing sleeve, and hence excessively raises the properties of development onto the photosensitive drum, thereby raising the density of solid images and the like. In addition, the weakly charged toner or reverse-polarity toner worsens a so-called fogging phenomenon in which thin toner is developed in a non-image portion on the photosensitive drum. This phenomenon is also conspicuous when images are printed on aforementioned glossy paper and the like. These phenomena are particularly notable after images are printed in a high-temperature, high-humidity environment.
To prevent the image deterioration and fogging phenomenon as described above, so-called developing device adjustment control is executed in various types of printers. For example, at every printing timing except for a printing operation, the high-triboelectrification toner is discharged from the developing sleeve (developing roller) and its vicinity in the developing device by developing the toner as a toner image such as a solid image on the photosensitive drum. Alternatively, the low-triboelectrification toner or reverse-polarity toner is agitated in the developing device by idling the developing sleeve (developing roller). When a control like this is executed, images can be printed by readjusting the average triboelectrification of the toner in the developing device to a preferred charge amount. Accordingly, various defective images caused by the developing device can be eliminated.
Note that the control for idling the developing sleeve (developing roller) is to idle the developing sleeve in a state in which no bias is applied to the developing device; in a state in which although a bias is applied to the developing device, the developing device is set at a potential equal to the potential on the photosensitive drum so that no toner is developed on the surface of the photosensitive drum; or in a state in which the developing sleeve is separated from the photosensitive drum in an arrangement in which the developing device and photosensitive drum can be separated.
Unfortunately, the developing device adjustment control described above poses new problems if the toner amount discharged from the developing device is not properly set. That is, in the conventional developing device adjustment control, the discharged toner amount is always set at a predetermined value regardless of the print log of the printer, in order to simplify the control. For example, if the discharged toner amount is small and insufficient, the average triboelectrification of the toner in the developing device cannot be readjusted to the preferred charge amount. Therefore, even after the developing device adjustment control is executed, the decrease in density of solid images or the deterioration of image quality of line images or characters continuously occurs. On the other hand, if the discharged toner amount is excessively large, the consumption of the toner in the developing device is accelerated, and this increases the running cost when the user uses the printer.
The above developing device adjustment control also poses new problems if the developing device idling time is not properly set. That is, in the conventional developing device adjustment control, the idling time is always set at a predetermined time regardless of the print log of the printer, in order to simplify the control. For example, if the idling time is short and insufficient, the average triboelectrification of the toner in the developing device cannot be readjusted to the preferred charge amount. Therefore, even after the developing device adjustment control is executed, the increase in density of solid images or fogging continuously occurs. On the other hand, if the idling time is excessively long, the downtime of the printer increases, and this decreases the throughput of the printer.